1. Field of the Invention
This invention relates to well cementing plug assemblies for use in a pipe such as casing during the cementing of the pipe in a well bore, and more particularly, to such plug assemblies which can support high pressure differentials without damage.
2. Description of the Prior Art
In the primary cementing of a well, a cement slurry is pumped downwardly through the pipe to be cemented in the well and then upwardly into the annulus between the pipe and the walls of the well bore. Upon setting, the cement bonds the pipe to the walls of the well bore and restricts fluid movement between formations penetrated by the well bore.
At the commencement of primary cementing, the pipe to be cemented and well bore are usually filled with drilling mud. In order to reduce contamination of the cement slurry at the interface between the drilling mud and cement slurry, a plug which includes a plurality of elastomeric wipers for sealingly engaging the inner surface of the pipe is pumped ahead of the cement slurry whereby the cement slurry is separated from the drilling mud as they are displaced through the pipe. The plug wipes the drilling mud from the walls of the pipe ahead of the cement slurry and maintains the separation between the cement slurry and drilling mud until it lands on a float collar or float shoe attached to the bottom end of the pipe.
The bottom plug can include a rupturable member or when it lands it can open a valve mechanism which allows the cement slurry to proceed through the plug and upwardly into the annular space between the pipe and the well bore. When the required quantity of the cement slurry has been pumped into the pipe, a top plug is released into the pipe to separate the cement slurry from additional drilling mud or other fluid used to displace the cement slurry down the pipe.
The design of the top plug is such that when it lands on the bottom plug it shuts off fluid flow through the plugs which prevents the displacement fluid from entering the annulus. When the top plug lands, the usual practice is to continue pumping the displacement fluid into the pipe whereby the pipe is pressured up and the pipe and associated equipment including the pump are pressure tested for leaks or other defects. A valve in the float collar or float shoe prevents the reverse movement of the cement slurry through the pipe. Once the cement has set, the top and bottom plugs are usually drilled out of the pipe.
The top and bottom cementing plugs can be released from an above ground cement plug container which is installed in communication with the interior of the pipe to be cemented. Such plug containers and their operation are well known to those skilled in the art. In an alternate arrangement, a sub-surface release plug assembly can be utilized which is positioned in the pipe to be cemented and attached to the lower end of a drill string suspended therein. Sub-surface release cementing plug assemblies are commonly utilized in sub-sea primary cementing operations. In such operations, the cement slurry is pumped through the drill string to the sub-surface release cementing plug assembly.
A problem which can be experienced when cementing plugs of either the above surface release type or the sub-surface release type are used involves the collapse or damage to the cementing plugs as a result of high differential pressures exerted on the plugs during the cementing operation, particularly during the pressure test described above. That is, when primary cementing is carried out in a well, the pressure differential exerted on a cementing plug can be as high as 15,000 psi or greater. Such high pressure differentials have heretofore caused the collapse or other damage to the cement plugs which prevent their proper operation and which can be very costly to correct. Thus, there is a need for improved well cementing plugs and assemblies which can withstand high pressures without damage to the plugs and/or causing the improper operation thereof.